Propeller wake fields in an open-water configuration were compared between two loading circumstances using large-eddy simulation (LES) with a computational domain of 48 million grids and an overset mesh technique. To validate the results of the numerical simulation, available experimental data are compared, which indicates that the grid systems are suitable for the present study. The results indicate that the present LES simulations describe the inertial frequency range well for both high and low-loading conditions. Under high-loading conditions, the interlaced spirals and secondary vortices that connect adjacent tip vortices amplify the effects of mutual inductance, ultimately triggering the breakdown of the propeller wake systems. At a great distance from the propeller, the vortex system loses all coherence and turns into a collection of smaller vortices that are equally scattered across the wake. In contrast, under light-loading conditions, the wake vortex system exhibits strong coherence and has a relatively simple topology. The elliptic instability and pairing processes are only observed at a far distance from the propeller. The convection velocity transferring tip vortices downstream is larger under the light-loading condition, which leads to the larger pitch of the helicoidal vortices. The larger pitch weakens the mutual inductance or interaction effects among tip vortices, which delays the instability behaviors of the whole vortex system. The results and implications of this study serve as a guide for the development and improvement of next-generation propellers that function optimally when operating behind aquaculture vessels.

The noise analysis of a large-scale aquaculture vessel reveals that during its navigation, the primary equipment noise, particularly from the propeller, exerts a notable influence on the aquaculture environment for large yellow croaker. The free surface greatly impacts the noise performance of propellers, which is a significant factor affecting the fish’s habitat. This study adopts the numerical simulation method to analyze the hydrodynamic and acoustic characteristics of the E1619 propeller operating near the free surface. The open-water performance and noise calculations of the propeller are verified through experiments, and the effects of different immersion depths and advance coefficients on the propeller are explored. The results demonstrate that the free surface significantly affects the thrust, torque, and noise of the propeller, especially at shallow immersion depths and low advance coefficients. Surface wave pattern causes the instability and breakup of tip vortices, causing increased thrust and torque fluctuations, reduced efficiency, and significant overall sound pressure levels in the entire flow field. As immersion depth and advance coefficients increase, the interaction between tip vortices and the free surface weakens, wake vortex instability decreases, and noise levels gradually reduce. These analyses and conclusions can guide the design of next-generation propellers for aquaculture vessels to optimize performance near the free surface.

Guoxin No. 1, one of the largest smart aquaculture vessels currently in operation, represents an important development in large-scale mobile aquaculture platforms. Continuous circulation of oxygen-rich seawater is essential for maintaining suitable culture conditions in intensive aquaculture vessels. However, the complex pipeline arrangements used for distributing seawater among multiple farming tanks can result in uneven flow allocation. In this study, three-dimensional CFD simulations were conducted to investigate the internal flow behaviors and flow distribution in a T-type pipeline system for an aquaculture vessel. Extending previous studies based on coplanar branch configurations, a non-coplanar skew branch pipe arranged in the xy-plane was investigated. Particular attention was given to the influence of the spatial offset between the branch pipe and the main pipeline axis on velocity distribution, pressure variation, and outlet flow rates. The results show that the spatial offset modifies the momentum transfer process at the junction and generates a swirling motion inside the main pipeline. Compared with the conventional inclined branch configuration, the offset branch shifts the high-velocity core away from the pipe center and reduces the imbalance between the two outlet flow rates. The pressure field is also redistributed due to the asymmetric momentum injection, indicating a trade-off between improved flow uniformity and local pressure variation. The present study demonstrates the potential of non-coplanar skew branch configurations as a passive approach for improving seawater distribution in large-scale aquaculture vessel pipeline systems.